Optical radiation detectors that utilize ultraviolet and infrared detectors for flame detection are used in many installations where a fast and reliable response to a fire is required. Various combinations of ultraviolet (UV) and infrared (IR) detectors are used such as UV only, UV and IR, dual IR, triple IR, and CCD or IR array cameras depending on the type of facility to be monitored and the environmental conditions. In locations where the flame may be the result of an explosion, such as in petrochemical plants and storage areas for flammable materials, the optical detector is enclosed in an explosion-proof housing. The area to be protected is viewed by the detector through an explosion-proof optical window typically made of quartz or sapphire.
In order for an optical flame detector to operate correctly, it is clearly necessary to ensure that the window is always sufficiently clean to enable the optical radiation detector element to receive the radiation to be detected. An arrangement to enable the cleanliness of the window to be checked is therefore required. This test needs to be performed at sufficiently close and periodic intervals and is referred to herein as Continuous Optical path Monitoring (COPM). The test also checks the operation of the optical flame detector and circuitry in addition to checking the cleanliness of the window. If a separate optical detector is used for the COPM test, this auxiliary detector will only check the cleanliness of the optical window but not the operation of the whole instrument.
The requirement for the cleanliness of the optical viewing window or lens is not limited to optical flame detectors, but is a general requirement for satisfactory operation of optical instruments such as infrared and optical cameras, imaging arrays, optical surveillance equipment and weapons. Cleanliness of the outermost optical component is a concern when the equipment is operated in a harsh industrial or military environment.
An arrangement using an external test lamp mounted on the housing to provide the optical illumination does not satisfy the requirements of certain industry specifications, which require that the test lamp be positioned inside the explosion-proof housing. In U.S. Pat. No. 4,529,881, the viewing window is recessed in a housing cavity with a flared wall. The test radiation is applied from a lamp in a portion of the housing that extends forward of the plane of the viewing window. This approach leads to complexity in design to ensure the explosion--proofing of the test lamp, increased expense in fabricating the housing, and interference of the protruding housing with the optical field of view of the flame detector.
Another approach, taken in U.S. Pat. No. 3,952,196, and U.S. Pat. No. 4,547,673, is to reflect light from a lamp inside the housing off a reflective metal ring or surface fixed to the outside of the housing. Such systems can give a false indication of impaired performance, as the test light passes twice through the viewing window, and must be reflected from a surface which may be corroded or covered with an accumulation of dirt.
Drawbacks of the reflective ring approach have been addressed in systems which utilize a lamp inside the housing that is positioned so that light is reflected back from the exterior surface of the optical window, by total internal reflection, onto the optical detector. In U.S. Pat. No. 4,405,234, the light is deflected by using beveled windows. In U.S. Pat. No. 4,826,316, the beveled window is replaced with a flat deflecting mirror to eliminate the high cost of the beveled window. The internal reflection approach works well for deposits of dust or contaminants directly on the window's exterior surface. However, blockage of the optical path due to non-deposits, such as a spider and spider web over the window, will not necessarily be detected. Since the optical window is recessed in the housing due to the explosion-proof requirements on the packaging this possibility is quite real. U.S. Pat. Nos. 4,405,234 and 4,826,316 illustrate the projection of the housing beyond the plane of the optical window. Additionally, the correlation between the deposit buildup on the window and the change in internally reflected light is difficult to characterize and quantify as the deposit in a rugged industrial environment can be of different materials with unknown optical properties. The internal reflection technique, therefore, does not test the actual optical path taken by the incoming radiation between the outside of the optical window and the edge of the housing.
It would therefore be an advance in the art to provide an optical flame detection apparatus with enclosed and protected elements to periodically check the transparency and cleanliness of the optical viewing window.
It would also be advantageous to provide an optical flame detection apparatus with enclosed and protected elements to periodically check the operation of the optical flame detector.
Yet another advantage would be to provide an optical instrument such as a camera or an imager with enclosed and protected elements to periodically check the transparency and cleanliness of the optical viewing window, and the operation of the instrument.
It would further represent an advance in the art to improve the optical field of view of the flame detector.
It would represent yet another advantage to illuminate the entire optical viewing window and optical radiation detector, and thereby check cleanliness of the entire window in contrast to checking only a section of the window.
Another advance in the art would be to provide for a flame detection apparatus that can operate in corrosive environments.